Fiber reinforced silicon carbide matrix composites having high temperature applications requiring thermal and environmental stability and good thermal shock resistance are commonly used for combustion and exhaust components in jet and rocket engines, ceramic burner inserts, and heat exchanger tubes. Composite carbide matrix materials are well known and some of which materials are disclosed in U.S. Pat. Nos. 5,275,984; 5,316,851; and 5,455,106, all of which are herein incorporated by reference.
The combustion and exhaust components, in order to serve their intended purpose, have to be operated at high temperatures and under mechanical and thermal stresses. In some cases, thermal stresses result from a temperature gradient through the thickness of the composite, making up the component, when one surface of the composite, serving as the component, sees a much high temperature than the other surface. The hot side of the component may be in compression and would benefit from reduced thermal expansion. The cold surface of the component may be in tension and would benefit from higher thermal expansion. It has been observed that prior art fiber reinforced silicon carbide matrix is often microcracked due to these stresses and oxygen penetrates to the interface of the fiber-matrix, through these microcracks. The interface, as well as the fibers, becomes oxidized thereby leading to the failure of these composites used as combustion and exhaust components. Also, hot surfaces (&gt;2000.degree. F.) of these combustion and exhaust components are subject to the loss of the silica protective scale by volatilization as SiO or Si(OH).sub.4.
It is desired that in order to make successful use of these silicon carbide matrix composites, matrix cracks should be sealed to prevent oxygen and corrosive gas ingress to the fibers and interfaces therein. Thus, it is important to grade the composition and generate various stress zones so that the matrix cracks would self heal or seal under high temperature operating conditions. These sealed cracks will stop the ingress of oxidants into the interior of the composites and thereby prevent catastrophic failure. Various processing techniques used in the prior art including hot pressing and sintering, chemical vapor infiltration, and polymer infiltration and pyrolysis are not useful in fabricating ceramic composites with graded composition and stress zones.
Accordingly, it is a primary object of the present invention to provide silicon carbide matrix composites having means for sealing matrix cracks so as to prevent oxygen and corrosive gas ingress to the ceramic fiber and interfaces making up the silicon carbide matrix composites that serve as high temperature components.
Further, it is an object of the present invention to provide silicon carbide matrix composites with graded compositions and stress zones.
It is another object of the present invention to provide silicon carbide matrix composites serving as combustion and exhaust components having a first side with high thermal expansion, oxidation resistance, and crack sealant means and a second side with low expansion means.
It is a still further object of the present invention to provide silicon carbide matrix composites having compositional patterns which lead to differential volumetric expansion/contraction yielding tailored stress zones and also having an oxidation behavior that differs across the thickness of the silicon carbide matrix composites and is tailored to the exposure temperature and environment.